Electromagnetic Fields in the Immediate Vicinity of a Tower Struck by Lightning

We present an analysis of the electromagnetic fields at very close range from a tower struck by lightning. The components of the electromagnetic fields are evaluated for observation points above and below the ground plane, characterized by a finite conductivity. The computations are obtained using a Finite Difference Time Domain (FDTD) method. The results are compared with those obtained by the same lightning strike to flat ground. In both cases, the return stroke channel and the elevated strike object are appropriately included and represented using engineering models. The underground electric field is predominantly horizontal with a negative polarity, and is markedly affected by the ground finite conductivity. The vertical underground electric field component is characterized by a bipolar wave-shape. The presence of a tower results in a significant decrease of the electric fields in the immediate vicinity of the tower. Unlike the case of a groundinitiated return stroke, the above-ground vertical electric field associated with a return stroke to a tall tower appears to be affected by the ground conductivity. Depending on the value of this latter, this component could exhibit an inversion of polarity.

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